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    //数字类型转化为字符串类型，值仍为数字类型时的值，但是类型变为字符串类型，可以使用+""来实现隐式的转化
    //String()来进行类型的强制的转化
    var num = "12345";
    var notNum = NaN;
    var str1 = num + "";
    var str2 =notNum + "";
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